(1) Field of the Invention
The invention is related to a composite laminate for connection to at least one attachment component in a load-introduction joint by means of a load-introduction component, said composite laminate comprising the features of the claims. The invention is further related to a load-introduction component for attachment of at least one attachment component to a composite laminate in a load-introduction joint, said load-introduction component comprising the features of the claims.
(2) Description of Related Art
A load-introduction joint generally defines a connection between at least two different components, wherein a load is introduced from at least one first component into at least one second component. For instance, in aerospace engineering such a load-introduction joint is amongst others embodied between a rotor blade and an associated rotor head of a rotor in a rotary-wing aircraft, wherein the rotor blade is joined to the rotor head by at least one suitable load-introduction component.
More specifically, in commonly used rotary-wing aircrafts the rotor blades are usually embodied as composite laminates made from composite materials, such as fiber reinforced polymers (FRP), which are joined to a respective rotor head by means of bolts. In operation of such a rotary-wing aircraft, loads acting on a given rotor blade are, thus, applied to the composite laminate defining the rotor blade and act on the composite laminate in essentially one main load direction that is defined by the longitudinal axis of the rotor blade. These loads are introduced from the composite laminate in the main load direction into the rotor head by means of at least one bolt, which defines the load-introduction component in the corresponding load-introduction joint.
The above described load-introduction joint is implemented as a bolted connection, as the load introduction into the composite laminate is performed by means of at least one bolt acting as load-introduction component. In such a bolted connection with a single main load direction, an underlying diameter d of the bolt, a diameter D of an opening provided in the composite laminate for reception of the bolt, a thickness t of the composite laminate, a distance e1 from the center of the opening to an edge of the composite laminate that is arranged in parallel to the main load direction, and a distance e2 from the center of the opening to an adjacent edge of the composite laminate that is arranged perpendicular to the main load direction, are considered as critical dimensions in the design of the bolted connection.
However, it should be noted that in cases, where the bolted connection comprises multiple bolts that are arranged in close relationship with respect to each other, only half of a given distance between the centers of two neighboring openings provided in the composite laminate are used for the determination of e1 or e2, depending on underlying relative positions of the neighboring openings on the composite laminate and the main load direction. Furthermore, in cases without a single main load direction, a smallest determinable edge distance e_min is considered.
The above described critical dimensions are considered during manufacturing of bolted connections, i.e. constituent components thereof. Conventional bolted connections with a composite laminate are usually realized according to one of two different main manufacturing techniques. These two different main manufacturing techniques are briefly described hereinafter.
The document JP 2012/061672 A describes a bolted connection that is embodied according to a first manufacturing technique with a composite laminate having fibers that are arranged in a loop around an opening in the composite laminate. The fibers are implemented as straight lines towards or away from the opening from or into the composite laminate. The opening is adapted to receive a load-introducing bolt, such that an elasto-mechanical mechanism is created which transforms radial forces resulting from a contact between the opening and the bolt into tangential forces in the composite laminate around the opening. Stresses that are directly resulting from these radial forces are referred to as “bearing stresses”, while stresses that are resulting from the tangential forces are referred to as “tangential stresses”.
In such a bolted connection that is embodied according to this first manufacturing technique, the orientations of the tangential stresses and the fibers are essentially parallel to each other, as the fibers are arranged in a loop around the opening. Thus, the composite laminate has its highest strength in parallel to the fiber orientations. Furthermore, in adequately designed loops the tangential stresses are larger than the bearing stresses and the elasto-mechanical mechanism that transforms the radial forces into the tangential forces results in shear stresses, which act between the fibers.
However, manufacturing of such a composite laminate having fibers that are arranged in a loop around an opening in the composite laminate is generally complicated, time-consuming and expensive, as it is difficult, and often impossible, to automate fiber placement during manufacturing, so that usually manual fiber placement is required. Furthermore, an available design space for such composite laminates is constrained, as usually comparatively small relative edge distances with e/D <2 are requested, so that a proper design of a composite laminate that is suitable to bear high loads would require large thicknesses t of the composite laminate.
The document US 2012/0045613 A1 describes a bolted connection that is embodied according to a second manufacturing technique with a composite laminate having multiple layers. Each layer comprises straight fiber orientations or is embodied as a fabric with two straight fiber orientations. More specifically, the fiber orientations in the multiple layers of the composite laminate are selected in order to bear the different load components described above, i.e. bearing stresses, tangential stresses and shear stresses. Accordingly, the fibers can be unidirectional, woven, knitted, braided, stitched, and so on.
In such a bolted connection that is embodied according to the second manufacturing technique, the composite laminate is provided with an opening that is adapted to receive a load-introducing bolt. The composite laminate as such is generally referred to as “bearing laminate”.
However, such a bearing laminate has associated failure modes that may occur abruptly in operation and, thus, affect safety of use of the bearing laminate. In order to increase the safety of use, an implementation of gradual failure modes is required. This can, e.g., be achieved by designing the bearing laminate with comparatively large relative edge distances of e/D>2˜3, depending on the type of bearing laminate, and small thicknesses t of the bearing laminate.
It should be noted that alternative manufacturing techniques of composite laminates for bolted connections are also known. Such alternative manufacturing techniques are, by way of example, described in the documents DE 10 2006 001 444 A1, DE 10 2006 007 428 A1 and DE 10 2010 009 769 A1. Furthermore, the above described first and second manufacturing techniques can be combined, so that composite laminates consisting of layers that are embodied as loops and layers that are embodied as bearing laminates are obtained.
However, all such manufacturing techniques are complicated, time-consuming and expensive, or result in composite laminates with comparatively large relative edge distances.